I believe the X4 narrow soapbar is the same as a J-bass pickups in a different shell (they are the same price). Most builders, probably, would rather use a more standard pickup rout thus the scarcity of the X4 soapbar. They do look cool, though, and I'm about to route for some X5's in a bass.

yeah, beside the sound, it's more of the look factor that I like about them. I also wanted my builder to make a wood cover for them and since the mounting hole on the X4 are on the side instead of top and bottom like the traditional jazz pickups, it will make things easier for him and the dimension of the final shape wouldn't change as much.